This invention relates to methods and processes for supplemental substrate treatment for biological control of algal metabolites and other organic contaminants. The new method may dose a process water stream with a readily biodegradable electron donor prior to treatment in a biological reactor.
Many current processes may use preozonation that may be an expensive process in biological treatment of algal metabolites in drinking water. Ozonation may form disinfection by-products that may be deleterious to human health. The presence of objectionable taste and odor compounds in surface water supplies may be a growing problem for water utility suppliers. Two common surface water compounds are 2-methylisoborneol and trans-1, 10-dimethyl-trans-9 decalol (geosmin), which are metabolites of cyanobacteria, blue-green algae, and actinomycetes bacteria.
Existing methods for mitigating algal metabolite impacts on drinking water may include algal population control through water reservoir management and metabolite removal at a water treatment facility. The growth of copper resistant algal strains and increased nutrient loading to surface waters may limit the effectiveness of reservoir management. Conventional treatment methods, such as pre- and post-chlorination, coagulation, sedimentation and filtration may be marginally effective at reducing algal metabolite concentrations.
Powdered activated carbon may be used in existing methods to supplement the processes to achieve acceptable removal of metabolites; although, such use may be cost prohibitive over lengthy or intense algal events. Some water processing utilities may ozonate settled water and then feed the water to biologically active granular media filters. The ozonation process may directly oxidize metabolites and may also oxidize natural organics to form assimilable organic carbon that may in turn provide a electron donor for microorganisms present in a biological filter. The ozone enhanced biofiltration process may be effective in water treatment, but may be costly and may have limited robustness, for example, diminished removal performance during fluctuations in feed water parameters.
Microorganisms may gain energy to grow and maintain cell metabolism by mediating the transfer of electrons between electron donor and electron acceptor. Primary electron donors may provide energy during cell metabolism. The rate of cell synthesis may be proportional to the concentration of a rate limiting electron donor, the cell yield, the concentration of active biomass, and the maximum specific primary electron donor utilization rate. The minimum concentration of primary electron donor that may support steady state biomass may be known as smin. When the rate limiting primary electron donor concentration equals smin, the rate of cell synthesis may equal the rate of cell decay. Any electron donor that may be present below its smin concentration may be known as a secondary electron donor. Though secondary electron donors may be biodegraded, bacteria may gain little to no energy in doing so, which may mean a primary electron donor may have to be biodegraded simultaneously. The rate of secondary electron donor degradation may be proportional to the concentration of active biomass present that may be a function of, among other factors, the concentration of primary electron donor.
Algal metabolites and other organic contaminants may be present in natural waters at parts per trillion or parts per billion concentrations and therefore may be biodegraded as secondary electron donors by indigenous microbial populations. Therefore, biological treatment processes designed to biodegrade these compounds may require the presence of a primary electron donor. The ozonation portion of an ozone enhanced biofiltration process may provide some direct oxidation of algal metabolites and may also break large natural organic matter molecules into smaller, more readily biodegradable organic molecules, thereby increasing the concentration of primary electron donors. The ozone enhanced biofiltration process may provide some success in removing algal metabolites from drinking water; however, potential disinfection by-product formation, lengthy bioacclimation time requirements, and inadequate removal efficiency and process robustness may limit full-scale use. A method that may provide a biological filter with an easily biodegradable primary electron donor at a controlled dose may allow a more efficient and robust process.